In freight transportation and mining railways, the loading weight is heavier than the loading weight on passenger coaches, and therefore the load applied to the axle shafts of freight cars is high and the environments at the areas of contact between rails and wheels are very rigorous. For use under such environments, rails are required to be wear resistant and steel having a pearlite structure is conventionally used. In recent years, freight and mineral loading weight is further increasing so as to enhance efficiency in rail transport, and therefore wear of rails is becoming more serious and the service lives of rails before replacement are decreasing. Because of this, improvement in the wear resistance of rails is demanded so as to enhance the service lives of rails before replacement. In addition to this, improvement in damage resistance is important, and a high level of ductility and a high level of toughness are also demanded.
Conventionally, many hard rails having improved rail hardness are developed. For example, Patent Literatures 1, 2, 3, and 4 disclose a hyper-eutectoid rail with an increased cementite content and a method for manufacturing the same. Patent Literatures 5, 6, 7, and 8 disclose a rail having a finer interlamellar spacing in a pearlite structure of eutectoid carbon steel so as to increase hardness.
In addition, many techniques are developed to increase hardness of rails by controlling conditions in manufacturing, such as rolling conditions and cooling conditions. For example, Patent Literature 8 discloses a technique that employs a cooling rate of 1° C./s to 10° C./s for the surface of a rail top starting at a temperature of equal to or more than Ar1 until pearlitic transformation occurs on the surfaces of the rail top and rail top lateral sides and then proceeds into a region at a depth of up to 5 mm from the surface, and then employs a cooling rate of 2° C./s to 20° C./s for the surface of the rail top until pearlitic transformation is completed in a region at a depth of 20 mm or greater from the surface.
Patent Literature 9 discloses a technique that carries out finishing rolling at a temperature of the surface of a rail top within the range of equal to or less than 900° C. and equal to or more than an Ar3 transformation point or an Arcm transformation point to achieve a cumulative surface area reduction rate of the rail top of equal to or more than 20% and a reaction force ratio of equal to or more than 1.25, and then subjects the surface of the rail top that has been subjected to finishing rolling to accelerated cooling or natural cooling at a cooling rate of 2° C./s to 30° C./s to a temperature of at least 550° C. Patent Literature 9 also discloses a rail having internal hardness at a depth of 2 mm from the surface of a rail top of HV 350 to HV 485 (HB 331 to HB 451), excellent ductility, and excellent wear resistance.
Patent Literatures 10, 11, and 12 disclose a technique to subject a rail top that has been subjected to finishing rolling to accelerated cooling and then, after raising the temperature and holding the temperature, perform another round of accelerated cooling.